The invention relates to a mixer arrangement comprising first and second mixer stages to which quadrature mixing signals are applied from an oscillator circuit, these two mixer stages being coupled to a superposition circuit.
Such a mixer arrangement is disclosed in the article "A third method of generation and detection of SSB-signals" by D. K. Weaver, published in Proceedings of the IRE, 1956, Vol. 44, No. 12, pages 1703-1705.
In some applications of prior art mixer arrangements, the frequency of the oscillator is located in the frequency range of the desired signal at the outputs of the two mixer stages, such as, for example, in single-sideband receivers based on the demodulation principle described in the above-mentioned article. In those cases, this desired signal can be seriously disturbed by parasitic oscillator cross-talk and/or d.c. off-set in the mixer stages. a suppression of these oscillator frequency-coupled interferences, called oscillator interference hereinafter for the sake of brevity, in the desired signal by means of notch filters results, especially in the above-described application of the mixer arrangement, in a signal component suppression which is audible to an annoying degree.
A feed forward compensation of the oscillator interference by means of a compensating signal derived from the oscillator signal, such as, for example, described in United Kingdom Patent Application No. 811,274, to compensate for oscillator radiation, only operates correctly for one given amplitude and phase of the oscillator interference. However, the amplitude and the phase of the oscillator interference vary and depend inter alia on the oscillator frequency, on the magnitude of said d.c.-offset of the relevant mixer stages and on the magnitude of the reactance of the undesired parasitic, usually capacitive couplings in or at the mixer stages, which can be infuenced to such an extent by constructional tolerances, temperature fluctuations, aging, etc., that they can, in practice, not be determined unambiguously.